Resistor not in schematic ... why ?

Thread Starter

xtal_01

Joined May 1, 2016
169
Just when I thought I was done.

Took the schematic ... after you guys answered a dozen questions, I made a parts list.

Then I took one more look at the board and saw a resistor R7

THERE IS NO R7 IN THE SCHEMATIC !!!!!!!!

Ok, so I traced it out.

I added it in read to the schematic.

It is smaller than the rest .. maybe 1/4 Watt ... Brown-Red-Black-Brown- Brown ... or the opposite.

So 1.2K ( or 11K if I am reading it backward).

Why is it in the circuit?

Really it is in parallel with the relay ,,, and draws down the positive line to the relay and green LED.

There is a trace for it and printing on the board saying R7 ... so it was designed into it, not added later.

Just not sure if I should add it to the drawing or eliminate it from the board.

Just FYI, I am guessing the jumper is for testing ... it could be pulled to simulate the pressure switch tripping.

Thanks!
added_resistor.jpg
 

Tonyr1084

Joined Sep 24, 2015
7,783
Many manufacturers will design a board with their "Premium Grade" possibility then only populate the board to "Economy Grade". Meaning there can be a lot of components not installed. I recall ONE board that had options for seven amplifier circuits but most commonly only one or two amp circuits were built. It's cheaper to have a single board designed that can do many different levels of functioning than it is to have seven different designed boards, one for each scenario. I've also seen boards built with through-hole resistors that will deliberately eventually cut one leg of one resistor. I've seen this most often in boards that display information either in American standards such as miles, pounds, etc. or European displaying in metric standards. It's not until the board or product is destined for service in a given area which decides which component will be clipped (one leg) from the circuit. Clipping only one lead means a user can reprogram the device by solder bridging that clipped component and then clip the other, thus changing from one standard to another. Still others may rely on that resistor to mean one standard and removing that from the circuit sets it up for the other.

R7 may have been an R&D thing where they introduced various resistances to simulate different conditions the product may experience. Once all unknowns have been accounted for then the decision is made whether to include or exclude R7 in your case.

No, you should probably not put a resistor in its place. It's unlikely going to make any difference OR it may cause the complete failure of the board. If you got it without R7 then it's probably by design. No sense in redesigning the board, revising the schematic and updating the BOM (Bill Of Material) just to exclude one resistor.
 

Thread Starter

xtal_01

Joined May 1, 2016
169
Many manufacturers will design a board with their "Premium Grade" possibility then only populate the board to "Economy Grade". Meaning there can be a lot of components not installed. I recall ONE board that had options for seven amplifier circuits but most commonly only one or two amp circuits were built. It's cheaper to have a single board designed that can do many different levels of functioning than it is to have seven different designed boards, one for each scenario. I've also seen boards built with through-hole resistors that will deliberately eventually cut one leg of one resistor. I've seen this most often in boards that display information either in American standards such as miles, pounds, etc. or European displaying in metric standards. It's not until the board or product is destined for service in a given area which decides which component will be clipped (one leg) from the circuit. Clipping only one lead means a user can reprogram the device by solder bridging that clipped component and then clip the other, thus changing from one standard to another. Still others may rely on that resistor to mean one standard and removing that from the circuit sets it up for the other.

R7 may have been an R&D thing where they introduced various resistances to simulate different conditions the product may experience. Once all unknowns have been accounted for then the decision is made whether to include or exclude R7 in your case.

No, you should probably not put a resistor in its place. It's unlikely going to make any difference OR it may cause the complete failure of the board. If you got it without R7 then it's probably by design. No sense in redesigning the board, revising the schematic and updating the BOM (Bill Of Material) just to exclude one resistor.
Thanks for the advice. Just was so odd to find it on the board ... even a silk screen showing it ... yet nothing on the drawing and for the life of me, I don't see what it does in the circuit.

My only guess was that it gave some kind of minimum load to the regulator ... but I don't see in the data sheets that you need one.

Thanks !!!!
 

MisterBill2

Joined Jan 23, 2018
17,848
Looking at the circuit, if that nearby "jumper" is not included then R7 would serve the purpose of supplying 12 volts across the pressure switch connection, for some unknown reason. OT if a voltmeter were connected across the "jumper connections and a pressure transducer were to replace the pressure switch, then R7 would be part of a pressure gage system. Omit R7 unless you are adding a pressure sensor and pressure gage.
 

Thread Starter

xtal_01

Joined May 1, 2016
169
Looking at the circuit, if that nearby "jumper" is not included then R7 would serve the purpose of supplying 12 volts across the pressure switch connection, for some unknown reason. OT if a voltmeter were connected across the "jumper connections and a pressure transducer were to replace the pressure switch, then R7 would be part of a pressure gage system. Omit R7 unless you are adding a pressure sensor and pressure gage.
I agree completely ... and how knows, maybe 30 years ago there was a transducer ... again, no one seems to know.

Thanks!
 
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